(A) Field of the Invention
The present invention relates to a multi-step gate structure and method for preparing the same, and more particularly, to a multi-step gate structure having an increased channel length by incorporating a semiconductor substrate with multi-step structure and method for preparing the same.
(B) Description of the Related Art
FIG. 1 illustrates a metal-oxide-semiconductor field effect transistor (MOSFET) 10 according to the prior art. The transistor 10 is an important basic electronic device including a gate 20 consisting of a semiconductor substrate 12, a gate oxide layer 14 and a conductive metal layer 16, and two doped regions 18 serving as the source and the drain in the semiconductor substrate 12 at two sides of the gate 20. The transistor 10 may further include a nitride spacer 22 positioned on the sidewall of the conductive metal layer 16 for isolating the conductive metal layer 16 from the other electronic devices on the semiconductor substrate 12.
As semiconductor fabrication technology continues to improve, sizes of electronic devices are reduced, and the size and the channel length of the transistor 10 also decrease correspondingly. The transistor 10 has been widely used in the integrated circuit; however, the decreasing of the size and the channel length of the transistor 10 results in a serious interaction between the two doped regions 18 and a carrier channel 24 in the semiconductor substrate 12 under the gate oxide layer 14 such that the controlling ability of the conductive metal layer 16 on the switching operation of the carrier channel 24 is reduced, i.e., causes the so-called short channel effect, which impedes the functioning of the transistor 10.